This invention relates to the field of ribbon loudspeakers.
During extensive experimentation with ribbon loudspeakers, I have discovered that by reducing the gap between the side edges of the ribbon and the magnetic poles to distances of about 5 mils, a substantial reduction in the degree of harmonic sound distortion is produced relative to larger gaps in the neighborhood of 15-20 mils. I have determined experimentally that if the gaps between the ribbon edges and the poles are about six to seven mils, the third harmonic distortion is only 2% at about 1,000 cycles per second. In contrast, gaps of about 15 mils produced third harmonic distortion of 7% at about 1,000 cycles per second. Further gap reduction to about 3 mils produced a third harmonic distortion of 0.3% at about 1,000 cycles. The relatively large gaps of about 15 mils cause a pumping action of the air passing through the gaps during ribbon vibration which in turn produces turbulence, which is responsible for the increased distortion of reproduced sound. In my U.S. Pat. No. 3,564,163, incorporated by reference herein, I discuss at the bottom of Column 2 and the top of Column 3 the function of the transverse corregations which are advantageously formed within the ribbon.
The transversely formed corregations discussed and illustrated in the above mentioned patent, although deemed desirable to provide resilience because they allow the ribbon to be deflected back and forth without being permanently stretched, create difficulty in attempting to maintain the above mentioned desirable gap in the neighborhood of about 5 mils. This is because the ribbon will laterally shift about or "snake" due to what I call an "accordian" effect owing to the corregations, and the "snaking" will cause the edges of the ribbon to strike the pole pieces which produces ribbon damage and undesirable sound distorting clattering. Furthermore, the striking of one pole piece by one ribbon edge portion during "snaking" will result in an enlargement of the gap at the opposite edge portion, producing the spillage of a significant body of air through the enlarged gap, to in turn generate the aforementioned detrimental turbulence.
Thus, it is an object of the present invention to enable the use of greatly reduced gaps of about 5 mils to dramatically reduce sound distortion by preventing the above mentioned lateral ribbon shifting or "snaking" phenomenon.
At the top of Column 3 of the above mentioned patent, I briefly suggest a stiffening of the central ribbon portions by "longtitudinal corregations or dimpling." The use of longtitudinal corregations, oriented perpendicular to the transverse corregations at the terminal portions, was attempted to eliminate "snaking" and was unsatisfactory because undue stress was set up at the discontinuous boundary between the transverse corregations and the longtitudinal corregations, which in turn caused serious ribbon damage; likewise with respect to the dimpling approach, which involved forming dimples in the centralized ribbon portion. I have also attempted to form transverse corregations at terminal ribbon portions and eliminate all corregations at the centralized portion. The result is crinkling of the ribbon with accompanying distortion of sound. By shaking a thin sheet of ordinary kitchen aluminum foil, this crinkling sound may be heard.